The present invention relates to a thermal imprint ink sheet repeatedly used in a thermal imprinting apparatus. The thermal ink sheet consists of an ink layer, a substrate and an anchor coated adhesive layer inserted between the ink layer and the substrate, and the present invention, in particular, relates to the anchor coated adhesive layer.
The thermal imprint ink sheet, which will be simply called "ink sheet" hereinafter, is used in a thermal imprinting apparatus for thermally imprinting data such as characters or graphics on a recording paper. The thermal imprinting is performed by inserting the ink sheet between the recording paper and a thermal head of the thermally imprinting apparatus, applying an electrical signal of the data to the thermal head. That is, when the ink sheet is heated by the thermal head in accordance with the electrical signal, an ink material is oozed from the ink layer and imprinted on the recording paper.
Before, the ink sheet was consisted of a substrate and an ink layer and used only once and thrown away after imprinting. However, since an anchor coated adhesive layer is applied to the ink sheet and the ink layer is improved, the ink sheet comes to be used repeatedly with economy.
FIG. 1 is a partially cross-sectional view of an ink sheet 4 which can be repeatedly used. As shown in FIG. 1, the ink sheet 4 includes a substrate 1 made of polyester resin, an thermally imprinting ink layer 3 and an anchor coated adhesive layer 2 inserted between the ink layer 3 and the substrate 1; wherein, an thermally imprinting ink layer and an anchor coated adhesive layer will be called simply an "ink layer" and an "adhesive layer" respectively hereinafter. As shown in FIG. 1, since the ink sheet 4 is inserted between a recording paper 6 and a thermal head 5 of a thermally imprinting apparatus, the substrate 1 and the recording paper 6 touch a thermal head 5 and the ink layer 6 respectively. Accordingly, when the thermal head 5 is heated up higher than 300.degree. C. in accordance with the electrical signal, the ink layer 3 is heated up higher than 100.degree. C. As a result, an ink material in the ink layer 3 is oozed out and imprinted on the recording paper 6. However, the ink layer 3 is kept staying in the ink sheet with the substrate 1 even though the ink layer 3 is heated up to 100.degree. C. by virtue of adhesiveness of the adhesive layer 2.
A Japanese Laid Open Patent No. SHO 57-105382 discloses about the adhesive layer. According to SHO 57-105382, the adhesive layer (of the prior art) is made of a fine powder consisting of resin and an inorganic material. Japanese Laid Open Patent SHO 59-166572 discloses about the ink layer. According to SHO 59-166572, the ink layer is made of an ink material having a low melting point, consisting of fatty acid amide, dyestuff and a filler material made of carbon black, and the ink layer is coated on a surface of the adhesive layer by using an organic solvent and dried up.
As mentioned above, the adhesive layer 2 is an indispensable element for the ink sheet to be repeatedly used. That is, in reference to FIG. 1, since the adhesive layer 2 is applied to the ink sheet 4, the ink layer 3 can be made adhere to the substrate 1 without peeling off from the substrate 1 even though the temperature of the ink sheet 4 is raised and lowered repeatedly in the imprinting. Every time the ink layer 3 is heated, the ink material impregnated in the filler of carbon black is oozed out and imprinted on the recording paper 6.
Since the thermal imprinting apparatus is allowed to operate at a room temperature from 10.degree. C. to 35.degree. C. in consideration of an environmental temperature, in winter and summer, around the apparatus, the adhesive layer 2 is required to have good flexibility and adhesiveness at the room temperature. To satisfy the requirement, there, is described in a Japanese Laid Open Patent SHO 60-49998 a mixed material of polyamide resin and polyester resin applied to the adhesive layer, so that the ink layer tightly adheres to the substrate. Wherein, the polyamide resin and the polyester resin in the adhesive layer well adhere to the fatty acid amide in the ink layer and the polyester resin in the substrate respectively.
However, the prior art ink sheet has problem such that: when the environmental temperature around the ink sheet decreases below the room temperature, the flexibility of the ink sheet is deteriorated, going so far as the ink layer happens to be peeled off from the substrate. Furthermore, if the above deterioration of the flexibility is improved by applying another material appropriate to the low environmental temperature to the ink sheet, the ink sheet produces a problem called "blocking", which will be explained later, in a fabricating process of the ink sheet.
As well known, a material being in a state of glass presents a phenomenon called "glass transition", and a temperature of the material presenting the glass transition is called a "glass transition temperature" which will be simply called a "Tg" hereinafter. In a case of the present invention, a Tg of the polyester resin in the adhesive layer is important. That is, if the temperature of the adhesive layer decreases below a Tg of the polyester resin in the adhesive layer, the adhesive layer becomes hard like glass and begins to loose its flexibility. On the contrary, if the temperature of the adhesive layer exceeds the Tg, the adhesive layer becomes soft and begins to be melted like paste as the temperature increases more. Therefore, when the adhesive layer includes a material having a Tg sufficiently low to maintain the flexibility at a low temperature, the adhesive layer becomes sticky at a high environmental temperature. This causes to occur the problem of blocking in a fabricating process of the ink sheet.
The ink sheet 4 in FIG. 1 is fabricated in accordance with two steps as follows, by using the same processing facilities 100 as shown in FIGS. 2(a) and 2(b):
(1) the substrate 1 is fabricated, which is not shown in FIGS. 2(a) and 2(b), and rolled to a first roll 8A shown in FIG. 2(a), then, using the processing facilities 100 as shown in FIG. 2(a), the substrate 1 is drawn out from the first roll 8A and led to a coating machine 7 in which the adhesive layer 2 is coated on a surface of the substrate 1 by using an organic solvent and dried up at a temperature of approximate 50.degree. C. to 60.degree. C., and the substrate 1 having the adhesive layer 2 coated thereon, which will be called an intermediate sheet 4' hereinafter, output from the coating machine 7 is rolled to a second roll 8B which is stocked for a while; and
(2) using the same processing facilities 100 as shown in FIG. 2(b), the intermediate sheet 4' produced in step 1) is drawn out from the second roll 8B and led to the coating machine 7 in which, in this time, the ink layer 3 is coated by using another organic solvent and dried at a temperature of approximate 50.degree. C. to 60.degree. C., producing the ink sheet 4 which is rolled to a third roll 8C.
In the above step (2), if the adhesive layer 2 of the intermediate sheet is sufficient sticky in the second roll 8B, the adhesive layer 2 will stick to a rear side, on which no adhesive layer is coated, of the substrate 1 of an inner side intermediate sheet 4' rolled in the second roll 8B. This problem is called "blocking" and occurs due to a low Tg of the polyester resin in the adhesive layer, compared with the environmental temperature, in which the coating the machine 7 is located.
To avoid the blocking problem occurring in step (2), it can be considered that the process of coating the ink layer 3 in the coating machine 7 in step (2), is performed immediately after the adhesive layer 2 is coated in the coating machine 7 in step (1), without stocking the intermediate sheet 4' in the second roll 8B. However, if the ink sheet is fabricated in such continuous process, two coating rooms 7 must be arranged in the processing facilities 100 and two layers (the adhesive layer 2 and the ink layer (3) must be coated at the same running speed. The coating machine 7 is very expensive and the two layers, which have a different composition and thickness and use different solvent individually, must be fabricated at the same running speed. Therefore, when the initial costs for the processing facilities and the complicated arrangement of fabrication conditions are considered, it must be concluded that the continuous process is not advisable.
The polyester resin in the adhesive layer is effective for adhering the adhesive layer to the substrate because the adhesiveness between the polyester resin in the adhesive layer and a polyester film of the substrate is excellent. Further, the polyester resin in the adhesive layer has a feature that the polyester resin is easily soluted by the solvent and has good compatibility with other resin materials in the adhesive layer. The other resin materials are for the adhesiveness to the ink layer. Thus, the polyester resin in the adhesive layer is very effective for obtaining excellent adhesiveness, flexibility and manufacturabilty.
However, in the prior art, only one kind of polyester resin is used in the adhesive layer. Therefore, the ink sheet of the prior art has a problem that if the polyester resin used in the adhesive layer has a low Tg, the blocking problem easily occurs, and if the polyester resin has a high Tg, the ink layer tends to be easily peeled off from the substrate when the ink sheet is used in a low environmental temperature, so that it becomes hard to use the ink sheet repeatedly.